Post-translational modifications by the small ubiquitin-like modifier (SUMO) family are important in oncogenesis and cellular response to DNA damage. Recent findings indicate that the key oncogenic pathways driven by Myc and KRas are dependent on, or addicted to, SUMOylation. In one study, genome-wide siRNA knockdown identified the gene encoding the catalytic subunit of the SUMO activating enzyme (SAE), SAE2, as having the strongest synthetic lethal interaction with Myc hyperactivation. Similarly, genes encoding the SUMOylation enzymes were found to be critical for KRas-dependent tumorigenesis. Based on these findings, we hypothesize that the SAE is a novel target for developing anti-cancer therapeutics for cancers that are Myc-and KRas-dependent. The over-arching goal of this proposal is to test this hypothesis, and the proposed studies are enabled by our discovery of potent and specific SAE inhibitors that have selective toxicity to cancer cell lines that have high Myc-expression levels and/or KRas mutation. We propose to investigate the molecular mechanisms underlying the synthetic lethality of SUMOylation with Myc hyperactivation and KRas mutations in order to further validate the SAE as a potential cancer therapeutic target. In addition, we will investigate the structure-activity relationship of how the inhibitors interact with the SAE and inhibit its enzymatic activity, and use this information to guide further improvement of the inhibitors. These studies could potentially improve treatment of many cancers, as overexpression of Myc is estimated to contribute to 70% of all human cancers, and KRas is also frequently mutated in human cancers. However, both Myc and KRas have proven difficult to inhibit pharmacologically. Thus, the proposed studies will likely establis a new paradigm to target the SAE for the development of novel cancer therapies.